A waveguide is a physical structure that guides the propagation of a wave. Waveguides may guide a variety of different types of waves, including electromagnetic waves. A slot waveguide is a waveguide formed by two closely spaced dielectric waveguides, such that for a wave polarized perpendicular to the slot the field maximum lies within the slot. Slot waveguides may be preferred over other waveguide types because slot waveguides may allow easier access to the optical field maximum at the center of the waveguide. In addition, slot waveguides allow stronger localization of the electric field inside the slot, or higher confinement, which is important for certain applications.
Fabrication of a slot waveguide is more difficult than the fabrication of conventional ridge waveguides. Slot waveguides are conventionally formed by cutting a narrow slot in a ridge waveguide. When a narrow slot is required, with widths of 20-100 nanometers (nm), a high resolution fabrication technique, such as electron-beam or nanoimprint (NI) lithography is used. The cutting process inherently produces rough sidewalls in the slot of the slot waveguide. The roughness in the sidewalls created in the vertical etching and lithographic processes causes wave propagation losses in the slot. A high degree of electric field localization is found at the vertically-etched sidewalls of conventional slot waveguides, which indicates that the propagation losses are greater than those of ridge waveguides, where the electric field localization is much lower. Experimental measurements have indicated losses in a Silicon-on-Insulator (SOI) slot waveguide of approximately 8 dB/cm, exceeding the <3 dB/cm values common to a SOI ridge waveguide. Moreover, conventional slot waveguides are expensive to produce because the high tolerances needed for accurate definition of the slot require etching and lithographic processes which are inherently difficult and costly.